Int J Legal Med DOI 10.1007/s00414-014-0970-8
REVIEW ARTICLE
Radioactive isotope analyses of skeletal materials in forensic science: a review of uses and potential uses Gordon T. Cook & Angus B. MacKenzie
Received: 21 November 2013 / Accepted: 21 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract A review of information that can be provided from measurements made on natural and anthropogenic radionuclide activities in human skeletal remains has been undertaken to establish what reliable information of forensic anthropological use can be obtained regarding years of birth and death (and hence post-mortem interval (PMI)). Of the anthropogenic radionuclides that have entered the environment, radiocarbon (14C) can currently be used to generate the most useful and reliable information. Measurements on single bones can indicate whether or not the person died during the nuclear era, while recent research suggests that measurements on trabecular bone may, depending on the chronological age of the remains, provide estimates of year of death and hence PMI. Additionally, 14C measurements made on different components of single teeth or on teeth formed at different times can provide estimates of year of birth to within 1–2 years of the true year. Of the other anthropogenic radionuclides, 90Sr shows some promise but there are problems of (1) variations in activities between individuals, (2) relatively large analytical uncertainties and (3) diagenetic contamination. With respect to natural series radionuclides, it is concluded that there is no convincing evidence that 210Pb dating can be used in a rigorous, quantitative fashion to establish a PMI. Similarly, for daughter/parent pairs such as 210Po/210Pb (from the 238U decay series) and 228Th/228Ra (from the 232Th decay series), the combination of analytical uncertainty and uncertainty in activity ratios at the point of death inevitably results in major uncertainty in any estimate of PMI. However, observation of the disequilibrium between these two daughter/parent pairs could potentially be used in a qualitative way to support other forensic evidence. G. T. Cook (*) : A. B. MacKenzie Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, Scotland, UK e-mail: [email protected]
Keywords Radiocarbon . Skeletal . Natural series radionuclides . Anthropogenic radionuclides . Post-mortem interval
Introduction One of the fundamental tasks often required of a forensic anthropologist is to establish the identity of deceased individuals based only on skeletal remains. The determination of age at death is particularly important as an accurate assessment will enable certain people to be eliminated from any missing persons enquiry while conversely, it may serve to highlight others that could require additional investigation. When dealing with the remains of juveniles, conventional determination of age at death can achieve the levels of accuracy required by the forensic anthropologist [1, 2]. However, age determination of adult remains is significantly less accurate, particularly in the post 40-year age range where the anthropologist is often only able to make limited statements such as ‘mature adult’ [3]. In adults, many procedures involving the examination of a range of skeletal characteristics have been proposed but unfortunately, most suffer from methodological bias and complex variability in the skeletal ageing process [4]. Even the best skeletal-based methods are often limited to the identification of broad age groupings [5]. In addition, time between death and discovery (post-mortem interval or PMI) can also be important in any investigation of human remains. There are a number of established techniques for estimating this but most are for relatively short-term intervals. Limitations in accuracy increase with increasing PMI and estimates based on bone morphology are strongly influenced by site factors throughout the PMI [6]. Radionuclides contained in human skeletal remains have some potential for estimating year of birth and year of death/ PMI because they decay at known, fixed rates and in some
Int J Legal Med
cases their levels can be related to man’s activities during known periods in time. During the last decade in particular, there have been a number of radionuclide studies designed to establish these parameters [6–16] and so the purpose of this review is to synthesise the relevant published work on the analysis of both natural and anthropogenic radionuclides in human skeletal remains and critically review their applicability in forensic science.
Discussion Radiocarbon (14C) Radiocarbon (14C) analysis is the most widely used technique for estimating either the year of birth or year of death of human remains. The traditional use of the radiocarbon dating method has been in archaeology where the technique has been applied to the dating of bone and teeth samples ranging between approximately 300 and 50,000 years. Here, the technique relies on a relatively constant rate of 14C production in the upper atmosphere. This is followed by rapid oxidation to 14 CO2, subsequent mixing with the stable carbon isotope forms (12CO2 and 13CO2) and uptake by green plants during photosynthesis, thus labelling all plant life with 14 C. Subsequent consumption of green plants by animals results in similar labelling of all animal life. All of these mixing and transfer processes occur very rapidly in comparison to the average lifetime of a 14C atom (8,300 years approx.) and so all living organisms are labelled to a first approximation with the same 14 C-specific activity [becquerel per kilogram (Bq kp-1)]. During life, an organism will retain this equilibrium living value, however, on death, it ceases to assimilate 14C and so the level decreases in accordance with its half-life. The radiocarbon age (i.e., the time that has elapsed since the organism died) is calculated according to: 1 Ao t ¼ ln λ At where t=time elapsed since death; λ=decay constant for 14C= ln(2)/half-life=1.245×10−4 year−1; A0 =equilibrium living value (based on measurement of a modern reference standard) and At =activity of the sample t years after death. N.B. The true physical half-life of 14C is 5,730 years but in radiocarbon dating the so-called Libby half-life of 5,568 years is used. This discrepancy is accounted for when calibrating radiocarbon ages to the calendar timescale. Since the late nineteenth century, man’s activities have influenced the atmospheric 14C concentration in two contrasting manners. First, the onset of the Industrial Revolution was accompanied by massive burning of fossil fuels. These contain no 14C because of their great age, and so releases of CO2
are confined to 12CO2 and 13CO2. This has resulted in a reduction in the atmospheric ratio of 14CO2:12CO2 (and 14 CO2:13CO2) such that from AD 1890 until the early 1950s, this dilution was measureable in annual tree rings from that period (Suess Effect) and by the late 1940s, this had resulted in an approximate 3 % reduction in the Northern Hemisphere [17]. Second, the atmospheric testing of nuclear devices (bomb effect) resulted in the production of 14C. These tests began in 1945 and continued until the Partial Test Ban Treaty in1963 which most countries with a nuclear capability signed up to. The tests led to an almost doubling of the atmospheric 14C activity of the Northern Hemisphere by 1963 and about a 65 % increase in the Southern Hemisphere. Since 1963, the atmospheric 14C activity has declined as the excess has entered the biota and the oceans [18]. Figure 1 illustrates the atmospheric 14C activity in the Northern Hemisphere between 1950 and 2010. The Suess Effect had a detrimental influence on conventional radiocarbon dating because it made it impossible to differentiate between organisms (including humans) that died in the period between approximately 1890 and 1950 and those that died between the mid-1600s and early-1700s. However, even in the absence of the Suess Effect, radiocarbon dating could not have provided the chronological precision required to make this a useful forensic technique for pre-1950 skeletal remains. One of the main reasons is the slow turnover of carbon in bone collagen, particularly during adulthood [19]. Other reasons include natural variations in the 14C production rate (Fig. 2), the error on the measurement and the very small annual reduction in 14C relative to this error. In contrast, the 14 C produced by atmospheric nuclear weapons tests has provided opportunities to study carbon turnover in collagen (and other tissues) and to provide significant information of forensic interest. Knight [20] stated that “no physico-chemical or morphological techniques have yet been devised that will determine date independently of environmental deterioration. The only exception is the radiocarbon estimation in bones of greater antiquity than those of medico-legal interest”. However, since then, significant advances have been made both in the measurement of radiocarbon and our understanding of carbon turnover in various components of skeletal remains. Radiocarbon measurements on bone collagen Through reference to the 14 C bomb peak it has been recognised that the 14C activity of human bone collagen lags significantly behind the activity in a range of organs and soft tissues [21, 22]. Hedges et al. [19] found that their data constrained models of collagen turnover in adult human femoral mid-shafts to ≤4 % between the ages of 20 and 80 years. During adolescent growth (10–15 years age), the turnover is
Int J Legal Med Fig. 1 Atmospheric 14C activity of the Northern Hemisphere during the period 1950–2010 expressed as a fraction of the natural equilibrium living activity
higher at 5 to 15 % year−1. Geyh [23] suggests a significant decrease in collagen turnover to 1.5 % year−1 after the age of 19 (termination of puberty). He produced a modelled relationship between year of death and the 14C activity of human bone collagen for several years of birth and plotted the measured 14 C values for individuals born with known dates of birth against these curves. The object here was to determine burial time. However, there were several outliers in the data and he concluded that burial time could seldom be estimated with a precision of
REVIEW ARTICLE
Radioactive isotope analyses of skeletal materials in forensic science: a review of uses and potential uses Gordon T. Cook & Angus B. MacKenzie
Received: 21 November 2013 / Accepted: 21 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract A review of information that can be provided from measurements made on natural and anthropogenic radionuclide activities in human skeletal remains has been undertaken to establish what reliable information of forensic anthropological use can be obtained regarding years of birth and death (and hence post-mortem interval (PMI)). Of the anthropogenic radionuclides that have entered the environment, radiocarbon (14C) can currently be used to generate the most useful and reliable information. Measurements on single bones can indicate whether or not the person died during the nuclear era, while recent research suggests that measurements on trabecular bone may, depending on the chronological age of the remains, provide estimates of year of death and hence PMI. Additionally, 14C measurements made on different components of single teeth or on teeth formed at different times can provide estimates of year of birth to within 1–2 years of the true year. Of the other anthropogenic radionuclides, 90Sr shows some promise but there are problems of (1) variations in activities between individuals, (2) relatively large analytical uncertainties and (3) diagenetic contamination. With respect to natural series radionuclides, it is concluded that there is no convincing evidence that 210Pb dating can be used in a rigorous, quantitative fashion to establish a PMI. Similarly, for daughter/parent pairs such as 210Po/210Pb (from the 238U decay series) and 228Th/228Ra (from the 232Th decay series), the combination of analytical uncertainty and uncertainty in activity ratios at the point of death inevitably results in major uncertainty in any estimate of PMI. However, observation of the disequilibrium between these two daughter/parent pairs could potentially be used in a qualitative way to support other forensic evidence. G. T. Cook (*) : A. B. MacKenzie Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, Scotland, UK e-mail: [email protected]
Keywords Radiocarbon . Skeletal . Natural series radionuclides . Anthropogenic radionuclides . Post-mortem interval
Introduction One of the fundamental tasks often required of a forensic anthropologist is to establish the identity of deceased individuals based only on skeletal remains. The determination of age at death is particularly important as an accurate assessment will enable certain people to be eliminated from any missing persons enquiry while conversely, it may serve to highlight others that could require additional investigation. When dealing with the remains of juveniles, conventional determination of age at death can achieve the levels of accuracy required by the forensic anthropologist [1, 2]. However, age determination of adult remains is significantly less accurate, particularly in the post 40-year age range where the anthropologist is often only able to make limited statements such as ‘mature adult’ [3]. In adults, many procedures involving the examination of a range of skeletal characteristics have been proposed but unfortunately, most suffer from methodological bias and complex variability in the skeletal ageing process [4]. Even the best skeletal-based methods are often limited to the identification of broad age groupings [5]. In addition, time between death and discovery (post-mortem interval or PMI) can also be important in any investigation of human remains. There are a number of established techniques for estimating this but most are for relatively short-term intervals. Limitations in accuracy increase with increasing PMI and estimates based on bone morphology are strongly influenced by site factors throughout the PMI [6]. Radionuclides contained in human skeletal remains have some potential for estimating year of birth and year of death/ PMI because they decay at known, fixed rates and in some
Int J Legal Med
cases their levels can be related to man’s activities during known periods in time. During the last decade in particular, there have been a number of radionuclide studies designed to establish these parameters [6–16] and so the purpose of this review is to synthesise the relevant published work on the analysis of both natural and anthropogenic radionuclides in human skeletal remains and critically review their applicability in forensic science.
Discussion Radiocarbon (14C) Radiocarbon (14C) analysis is the most widely used technique for estimating either the year of birth or year of death of human remains. The traditional use of the radiocarbon dating method has been in archaeology where the technique has been applied to the dating of bone and teeth samples ranging between approximately 300 and 50,000 years. Here, the technique relies on a relatively constant rate of 14C production in the upper atmosphere. This is followed by rapid oxidation to 14 CO2, subsequent mixing with the stable carbon isotope forms (12CO2 and 13CO2) and uptake by green plants during photosynthesis, thus labelling all plant life with 14 C. Subsequent consumption of green plants by animals results in similar labelling of all animal life. All of these mixing and transfer processes occur very rapidly in comparison to the average lifetime of a 14C atom (8,300 years approx.) and so all living organisms are labelled to a first approximation with the same 14 C-specific activity [becquerel per kilogram (Bq kp-1)]. During life, an organism will retain this equilibrium living value, however, on death, it ceases to assimilate 14C and so the level decreases in accordance with its half-life. The radiocarbon age (i.e., the time that has elapsed since the organism died) is calculated according to: 1 Ao t ¼ ln λ At where t=time elapsed since death; λ=decay constant for 14C= ln(2)/half-life=1.245×10−4 year−1; A0 =equilibrium living value (based on measurement of a modern reference standard) and At =activity of the sample t years after death. N.B. The true physical half-life of 14C is 5,730 years but in radiocarbon dating the so-called Libby half-life of 5,568 years is used. This discrepancy is accounted for when calibrating radiocarbon ages to the calendar timescale. Since the late nineteenth century, man’s activities have influenced the atmospheric 14C concentration in two contrasting manners. First, the onset of the Industrial Revolution was accompanied by massive burning of fossil fuels. These contain no 14C because of their great age, and so releases of CO2
are confined to 12CO2 and 13CO2. This has resulted in a reduction in the atmospheric ratio of 14CO2:12CO2 (and 14 CO2:13CO2) such that from AD 1890 until the early 1950s, this dilution was measureable in annual tree rings from that period (Suess Effect) and by the late 1940s, this had resulted in an approximate 3 % reduction in the Northern Hemisphere [17]. Second, the atmospheric testing of nuclear devices (bomb effect) resulted in the production of 14C. These tests began in 1945 and continued until the Partial Test Ban Treaty in1963 which most countries with a nuclear capability signed up to. The tests led to an almost doubling of the atmospheric 14C activity of the Northern Hemisphere by 1963 and about a 65 % increase in the Southern Hemisphere. Since 1963, the atmospheric 14C activity has declined as the excess has entered the biota and the oceans [18]. Figure 1 illustrates the atmospheric 14C activity in the Northern Hemisphere between 1950 and 2010. The Suess Effect had a detrimental influence on conventional radiocarbon dating because it made it impossible to differentiate between organisms (including humans) that died in the period between approximately 1890 and 1950 and those that died between the mid-1600s and early-1700s. However, even in the absence of the Suess Effect, radiocarbon dating could not have provided the chronological precision required to make this a useful forensic technique for pre-1950 skeletal remains. One of the main reasons is the slow turnover of carbon in bone collagen, particularly during adulthood [19]. Other reasons include natural variations in the 14C production rate (Fig. 2), the error on the measurement and the very small annual reduction in 14C relative to this error. In contrast, the 14 C produced by atmospheric nuclear weapons tests has provided opportunities to study carbon turnover in collagen (and other tissues) and to provide significant information of forensic interest. Knight [20] stated that “no physico-chemical or morphological techniques have yet been devised that will determine date independently of environmental deterioration. The only exception is the radiocarbon estimation in bones of greater antiquity than those of medico-legal interest”. However, since then, significant advances have been made both in the measurement of radiocarbon and our understanding of carbon turnover in various components of skeletal remains. Radiocarbon measurements on bone collagen Through reference to the 14 C bomb peak it has been recognised that the 14C activity of human bone collagen lags significantly behind the activity in a range of organs and soft tissues [21, 22]. Hedges et al. [19] found that their data constrained models of collagen turnover in adult human femoral mid-shafts to ≤4 % between the ages of 20 and 80 years. During adolescent growth (10–15 years age), the turnover is
Int J Legal Med Fig. 1 Atmospheric 14C activity of the Northern Hemisphere during the period 1950–2010 expressed as a fraction of the natural equilibrium living activity
higher at 5 to 15 % year−1. Geyh [23] suggests a significant decrease in collagen turnover to 1.5 % year−1 after the age of 19 (termination of puberty). He produced a modelled relationship between year of death and the 14C activity of human bone collagen for several years of birth and plotted the measured 14 C values for individuals born with known dates of birth against these curves. The object here was to determine burial time. However, there were several outliers in the data and he concluded that burial time could seldom be estimated with a precision of
